searches for long-lived particles with atlasnov 12, 2015 s. pagan griso 8 experimental challenges...
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Simone Pagan GrisoLawrence Berkeley National Lab.
Amherst Center for Foundamental InteractionsUMass Amherst,
November 12th 2015
Searches forlong-lived particles with ATLAS
LHC Searches for Long-Lived BSM Particles: Theory Meets Experiment
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2Nov 12, 2015 S. Pagan Griso
Introduction & Outline● Signature-based review of BSM long-lived particles (LLP) searches
in ATLAS
● Based on √s=8 TeV results (~all published, references therein)– Some earlier results may still be relevant(?), but not covered here
● Highlight experimental challenges and strengths of the detector
● No attempt to give details for each specific analysis– Especially if covered by a dedicated talk later in the workshop– List examples of benchmark models used, but won't be
comprehensive– Highlight (attempt of) model-independence when possible
● Ending with a quick look at the future
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3Nov 12, 2015 S. Pagan Griso
Hunting LLP
Direct detection● Through direct interaction with
detector● Energy loss, TOF, special track
properties, …● Mostly fit charged LLP
Indirect detection● Through SM or invisible decay
products● “Isolated” activity inconsistent
with prompt or expected instrumental / SM
● Natural fit for neutral LLP, but also sensitive to charged ones
● Various sub-detectors are sensitive to different life-time ranges
Det
ecto
r
Det
ecto
r
LLP
LLP
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4Nov 12, 2015 S. Pagan Griso
Hunting LLP
Direct detection● Plot: lifetimes for which
20% decays after the outer radius of the sub-detector
MuonSpectrometer
(MS)
Inner Detector (ID)
Calorimeters
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5Nov 12, 2015 S. Pagan Griso
Hunting LLP
Direct detection● Plot: lifetimes for which
20% decays after the outer radius of the sub-detector
Inner Detector (ID)
MuonSpectrometer
(MS)
Calorimeters
Indirect detection● Plot: lifetimes for which 20% of
decays within the inner/outer radius range of the sub-detector
ID
Calo
ID
MS
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6Nov 12, 2015 S. Pagan Griso
The (non-obvious) ATLAS detectorIonization loss: charge measured by:• Pixel system• Transition-Radiation Tracker (TRT)• Monitored drift-tubes (MDT) in the muon system
Inner Detector(ID)
Muon spectrometer(MS)
Time of flight: time of arrival by• Electromagnetic (EM) and Hadronic Calorimeters• Muon system
EM Calorimeter
Hadronic Calorimeter
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7Nov 12, 2015 S. Pagan Griso
Outline of ATLAS searches
Directdetection
Indirectdetection
ID Calo MS
Disappearing Track
Large ionization deposits
Time-of-flight measurements
Prompt analysis (jets+ET)
Displaced vertices
Non-prompt/delayed photons
“Isolated” jets
Collimated lepton-jets
Primary measurement
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8Nov 12, 2015 S. Pagan Griso
Experimental challenges● Some signatures can't be exploited at trigger-level directly
– Need rely on “collateral” features of the event (e.g. ISR, ET, ..)
– Develop dedicated trigger chains
● Many analyses targeting LLP require “non-standard” techniques– Detailed (low-level) detector information– Specific tracking setup to reconstruct
highly displaced tracks– Custom vertexing algorithms for
very displaced vertices– Careful balance of CPU timing and
disk-space required
● Detector efficiency model-dependent– Limit by careful choice of fiducial region– What benchmark to provide– How to allow re-interpretation / boundaries?
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9Nov 12, 2015 S. Pagan Griso
Disappearing track● Charged particle decaying within the ID into un-detected products
– Manifest as “short” track
● Requires hard ISR to trigger: ET+jet
● Isolated high-pT (> 75 GeV) track
with at most few hits in the TRT– Dedicated tracking setup required
● Results by fitting pT spectrum
– Main background at high-pT:
mis-measured tracks (data-driven)
● Signal model: AMSB SUSY
● 2-tracks signal region investigated
DirectDetection
PR D90 055031 (2014)
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10Nov 12, 2015 S. Pagan Griso
Large ionization tracksDirectDetection
EPJ C75 407 (2015)
● Heavy (→ slow) charged particles produce large ionization loss– Pixel detector provides accurate measurement; stability with p mass
● Trigger through calorimeter ET
● Isolated high-pT (>80 GeV) track
with large energy loss (dE/dx)– Background from hadrons and leptons
in the tails of dE/dx (data-driven)
● Limits set for benchmark scenarios:– Stable R-hadrons, “stable” – R-hadrons varying lifetime/mass/decay
● Decay hypothesis vary limits to some extent– (same as disappearing tracks)
● vs mass, mass vs lifetime
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11Nov 12, 2015 S. Pagan Griso
Large ionization tracks● Multiply charged particles also produce larger ionization loss
DirectDetection
EPJ C75 362 (2015)ArXiv:1509.08059 (PRD)
● Search for multiply-charged particles– Stable within ATLAS detector
● Combine Pixel, TRT and muon dE/dxmeasurements, depending on charge– Note: energy loss in calorimeter z2 (q=ze)
● Benchmark: Drell-Yan like pair production– vs mass limits; q=z*e, z=2-6;
● Search for monopoles– stop in EM calorimeter (special trigger)
● Analysis based on TRT high-threshold hits,no energy in calorimeter past first layers (w)
● Fiducial phase space defined to haveuniform and > 90% efficiency– depends on material traversed
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12Nov 12, 2015 S. Pagan Griso
Time-Of-Flight● Heavy charged particles travel with =v/c < 1 → detect with TOF
– Combine with information on ionizationloss (dE/dx from pixel detector) →
● Average time measurements fromcalorimeter and muon system– Calibrated using Z → , ad-hoc
tracking setup to correctly associate hitsin cased where << 1
● Trigger on single-muon or calorimeter ET
● Background mainly from muons with mis-measured or high dE/dx– Mostly rejected requiring consistency among independent detectors– Residual background estimated from random combination from the
and momentum distributions measured in suitable regions
DirectDetection
JHEP 01 068 (2015)
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13Nov 12, 2015 S. Pagan Griso
Time-Of-Flight● Three sets of signal regions, selections targeted for each one
DirectDetection
JHEP 01 068 (2015)
● Charginos nearly mass degeneratewith LSP (almost pure neutral wino)– Investigate 1 and 2-tracks signal; expect
significant ET when production
● Sleptons in GMSB ( NLSP) orLeptoSUSY simplified model( degenerate, LSP)– Investigate 1 and 2-tracks signal
in both ID and muon system
● Squarks and gluinos forming R-hadrons– Use full detector or ignore muon system
(charge → neutral interacting with calorimeter)– Optimized separately for gluino/stop/sbottom
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14Nov 12, 2015 S. Pagan Griso
Summary plots● Example of summary plot for a specific (SUSY) benchmark model● Useful for us for a quick overview, but need consistent benchmarks
DirectDetection
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15Nov 12, 2015 S. Pagan Griso
IndirectDetection Prompt-analyses re-interpretation
ATLAS-CONF-2014-037
● Re-interpret prompt analyses to targetgluinos with ~short lifetime (~ < 1ns)– Actual sensitivity extends also for longer lifetimes
● Standard jets+ET analysis provides significant constraints
● Lepton and b-jet identification non-optimal for displaced decay
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16Nov 12, 2015 S. Pagan Griso
● Multi-track: displaced vertexwith either one e,E
T or jets
● Di-lepton: displaced vertex from opposite charge ee,
Displaced vertices● Aim to reconstruct explicitly displaced decays of LLPs
– Ad-hoc tracking (large d0 tracks), veto known material (had. interactions)
– Background: accidental crossing, merged vertices, jets punch-through
IndirectDetection
● Dedicated muon-trigger for displaced vertices in the MS– Also using jet+E
T trigger
● Two-vertices signal region– Aim for large efficiency
Inner-Detector analysis
PR D92 072004 (2015)PR D92 012020 (2015)
Inner-Detector + Muon system analysis
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17Nov 12, 2015 S. Pagan Griso
Displaced verticesIndirectDetection
PR D92 072004 (2015)PR D92 012020 (2015)
● RPV, GMSB: (N)LSP long-lived● Split-SUSY: into R-hadron
Inner-Detector analysis Inner-Detector + Muon system analysis
● Non-trivial efficiency dependence on mass, boost, multiplicity, …– Weak dependence on originating particle given the above
● Hidden Valley, or Z' mediator● Stealth SUSY
Results presentedfor benchmarkmodels as function of- lifetime- mass spectrum- final state
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18Nov 12, 2015 S. Pagan Griso
Summary plot
● Nice complementarity of analyses shown in example SUSY benchmark summary plot (gluino R-hadron)
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19Nov 12, 2015 S. Pagan Griso
Non-prompt photons
● 2 photons pointing away from interaction and delayed in time– Additionally require E
T (>75 GeV), low E
T region as control region
● Main backgroundfrom prompt ,jets
● Use calorimetertiming (t) and pointing
from shower profile (zDCA
)
IndirectDetection
JHEP 11 088 (2014)
Exponentialdue to acceptance
of decay beforecalorimeter
Smaller ET
, ET
● GMSB(SPS8), NLSP● Limits on #events,
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20Nov 12, 2015 S. Pagan Griso
Jets in hadronic calorimeterIndirectDetection
PL B743 15 (2015)
● Narrow jets in hadronic calorimeter with no activity before (EM+ID)– Well fit neutral LLPs, complements
direct vertex reconstruction● Dedicated trigger: narrow (R=0.2) jets
with large EHAD
/EEM
and no activity in ID
● >= 2 jets required offline, low ET
– Loose timing requirement limitsacceptance in (OK, for benchmark)
● Hidden Valley benchmark modelwith scalar communicator
● Acceptance 0.07%-0.6%, dominatedby requiring both decays in calorimeter
● Limits on ()xBR( → v
v) for
m, m(v),
v) hypotheses
– Efficiency map vs v boost
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21Nov 12, 2015 S. Pagan Griso
“Lepton”-jets● Collimated cluster of /e/ (=”jets”)
– Trigger: multi- or jets in had. calorimeter– “Lepton”-jet types: 4, 2 + 2 ”jets”, 4 ”jets”
● e/ “jets” ~as previous slide– Sensitive to decays in had. calorimeter
● Muons reconstructed in MS only for high efficiency when displaced● Multi-jet and cosmic rays backgrounds data-driven
● Benchmark model: Higgs decayingto hidden sector, displaced decayof a light dark-photon (
D)
– H → 2D+X, H → 4
D+X
– Efficiency depends strongly on numberof
D, decay length and angular distribution
IndirectDetection
JHEP 11 088 (2014)
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22Nov 12, 2015 S. Pagan Griso
Very-long lived R-hadrons● Use bunch-structure of LHC: R-hadrons that decay in the
calorimeter after a “long” time (jet+ET from )
● Dedicated trigger; >= 1 calorimeter jet (but <=5), no MS activity– Additionally, E
T and loose jet shape requirements
● Main backgrounds: cosmic rays, beam halo● Different models of R-hadrons and gluino-neutralino M probed
IndirectDetection
PR D88 112003 (2013)
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23Nov 12, 2015 S. Pagan Griso
(Last) summary plot
● Very broad overview of lifetime sensitivity ranges
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24Nov 12, 2015 S. Pagan Griso
A brief look at the future
● Obvious: 7 TeV → 13 TeV :-)● Key upgrades can help in LLPs searches
– New pixel layer: IBL● E.g. better dE/dx measurement
– L1 Topological trigger→ correlation of L1 trigger primitives
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25Nov 12, 2015 S. Pagan Griso
A brief look at the future
● Obvious: 7 TeV → 13 TeV :-)● Key upgrades can help in LLPs searches
– New pixel layer: IBL– L1 Topological trigger
– FTK to be commissioned during Run-2→ full-tracking in-between L1 and HLT
● but tracks reconstructed tightlypointing to interaction region
– Software and tracking algorithmsspeed-up a factor of 4, re-investpart of the gain
Long Shutdown 1
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26Nov 12, 2015 S. Pagan Griso
Conclusions (/Opening)
● A variety of complementary methods used in run-1 to hunt for LLPs
● Many of these analyses used the ATLAS detector in unique ways– Always looking for new innovative ways to hunt signatures that are
theoretically well motivated and presently not fully covered
● Already towards end of Run-1, more systematic attempt to provide efficiency maps with results in simplified/benchmark models– However, very large model dependences are often found– How to specify “boundaries”?
● Looking forward to a fruitful workshop!
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27Nov 12, 2015 S. Pagan Griso
Backup
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28Nov 12, 2015 S. Pagan Griso
EM Calorimeter
Inner Detector
Hadronic calorimeter
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29Nov 12, 2015 S. Pagan Griso
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30Nov 12, 2015 S. Pagan Griso